withCurrentValueFrom (withLatestFrom)

Samples the current value from another observable each time the source emits. Emits a tuple of [sourceValue, sampledValue].

Marble Diagram

withCurrentValueFrom(obs2$)

Example

//  Timeline:
//
//  name$     "Alice"           "Bob"               "Charlie"
//  age$                25              30
//  result$   (skip)          ["Bob",25]           ["Charlie",30]
//
//  Explanation:
//  - withCurrentValueFrom samples the current value from another observable
//  - Emits a tuple [sourceValue, sampledValue] each time the SOURCE emits
//  - Does NOT emit when the sampled observable (age$) emits
//  - Does not emit until both observables have emitted at least once
//  - "Alice" is skipped because age$ hasn't emitted yet

const name$ = source<string>();

const age$ = source<number>();

const result$ = name$.pipe(withCurrentValueFrom(age$));

const valueHistory: (readonly [string, number])[] = [];

result$.subscribe(([name_, currentAge]) => {
    valueHistory.push([name_, currentAge]);
});

name$.next('Alice'); // nothing logged (age$ hasn't emitted)

assert.deepStrictEqual(valueHistory, []);

age$.next(25);

name$.next('Bob'); // logs: Bob is 25 years old

assert.deepStrictEqual(valueHistory, [['Bob', 25]]);

age$.next(30);

name$.next('Charlie'); // logs: Charlie is 30 years old

assert.deepStrictEqual(valueHistory, [
    ['Bob', 25],
    ['Charlie', 30],
]);

View source on GitHub